An illustrative but non-restrictive application of the present invention relates to location estimation for positioning tags, or briefly “tags”. In the present context tags are electronic devices whose principal purpose is to assist estimating the location of more valuable entities co-located with the tags, such as humans or pieces of equipment.
In a representative implementation the tags are operable to measure signal strengths from one or more base stations in a Wireless Local Area Network (WLAN) and to relay the signal strength measurements to a positioning application. In the following, the term positioning engine refers to a combination of the positioning application and the computer apparatus which executes the positioning application. Such techniques are disclosed in commonly-owned patent applications and patents, some of which are listed at the end of the description of the present invention.
In order to estimate a tag's position with sufficient accuracy and reliability, which typically means determining the room where tag is located, the tags should repeat signal strength measurements with sufficient repetition rate, such as with 10-second intervals or less and with sufficient scope, such that one observation (scan of measurable frequencies/channels) includes signal strength measurements from five or more base stations. A large number of measurements is required because signal strengths are subject to strong random fluctuations even in cases where the tag stands perfectly still. The random fluctuations are typically caused by radio-frequency interference and persons or objects moving in the environment, thus causing temporary attenuation in signal propagation. Because of the fluctuations, some of the tag's observations are incomplete, which means that signals from distant or heavily attenuated base stations could not be measured. An observation which lacks signal strength measurements from several base stations may result in gross positioning errors. Such errors are best eliminated by observing several base stations with a high repetition rate.
Unfortunately the requirements to measure several base stations frequently tends to increase battery consumption in the tag. The need to increase the tags' battery lifetime and to lower their manufacturing costs forces manufacturers to settle for a compromise that optimally balances the conflicting requirements of accuracy, cost and battery lifetime.
One strategy for extending battery lifetime is to enable the tag to spend most of the time in a sleep mode wherein most of its circuitry is shut off. The tag's normal operation is resumed in response to a timer alert, motion detection by a motion sensor, or the tag may have a pushbutton for that purpose. When the tag wakes up for a scanning operation, it sends a probe request message to one WLAN channel at a time and remains on that channel for some time in order to receive responses from the base stations. Each base station, which serves the channel on which the probe request message was sent and which is able to receive the message, responds by sending a probe respond message. If the tag is able to receive a base station's probe respond message, it is able to measure that base station's signal strength in the place where the tag is located. After scanning all pre-configured channels, the tag compiles the signal strength measurements from all base stations and sends the compiled signal strength measurements to the positioning engine which determines or updates the tag's location estimate. After this, the tag re-enters sleep mode. The sleep mode is an effective strategy for extending the tag's battery life, but it causes problem in finding the tag during sleep mode.
Another positioning problem is caused by the fact that the bandwidth requirements of WLAN networks or some sections of them may be adequately covered by one or two base stations but such a small number of observable base stations is insufficient for accurate and reliable positioning. This means that although, in principle, the tags can be positioned by using signals of base stations which exist for the sake of communication, it is not uncommon that existing WLAN networks must be complemented by adding and/or relocating base stations to meet the requirements of accurate and reliable positioning. A further problem is that the effect of the added and/or relocated base stations is difficult to predict a priori, and the base station placement tends to require experimentation which in turn requires labour-intensive recalibration of the data model.
Yet another positioning problem is caused by the fact that a tag's measurements may be more or less permanently hindered by obstacles. For instance, a tag put in a metal locker may not even attach to any base stations, which prevents it from sending its observations even if the tag is able to make observations. Such a tag may be completely lost until it is taken out from the metal locker.
Yet another positioning problem is caused by the fact that a tag may be in a place which has no WLAN network coverage. For instance, the network may suffer from design errors and or malfunctioning base stations which cause “dead spots”, ie, spots that are not adequately covered by any of the base stations. Alternatively or additionally the tag may be accidentally or intentionally moved outside the network coverage area. For example, a tag may be attached to a patient in a mental hospital and the patient may try to escape the hospital by leaving the building and moving outside the hospital network.
Yet another positioning problem occurs when using ad-hoc networks wherein only a few base stations are placed on fixed positions and the network clients communicate with each other in order to establish a communication network. In this scenario, most of the network clients may be outside the coverage area of the fixed access points. For instance, tags may be used to locate firefighters entering a building in fire, wherein wireless devices carried by the firefighters form an ad-hoc network which is connected to a few fixed base stations residing in fire trucks parked around the building. Thus, wireless devices near the fire trucks communicate to the base stations directly while other devices deeper in the building communicate via other devices.